1. Field of the Invention
The invention relates to ring or serial loop data communication systems and, more particularly, to method and apparatus for acquiring information relative to the ordering and for the precise location of active stations on the ring. The information may be used to correct a fault should one occur on the ring.
2. Prior Art
The use of serial or ring communication systems for transferring data is well known in the prior art. Such systems include a ring transmission medium to which a plurality of stations are connected. Access to the ring is controlled by a set of protocols which ensures that each station is given an opportunity to transmit and/or receive data from the ring.
Prior art ring communication systems may be classified into two groups, the centralized systems ad the decentralized systems. In the centralized systems, the ring is controlled from a centralized station. U.S. Pat. No. 3,564,145 describes a serial loop data transmission system having a central station with a computer (central control unit) for controlling the loop. Because most of the control or supervisory functions are handled through a single station, the overhead traffic between terminal stations and the central station is relatively high. This in turn may reduce the amount of useful data which is transmitted on the ring.
In the decentralized system the control or supervisory functions are distributed to the stations on the ring. Oftentimes a monitor station may provide functions such as clocking, generating frame headers, error checking and recovery, etc., but does not have central control. Such a decentralized system appears to be very attractive in that the communication link can support a relatively large number of stations or nodes with maximum data throughput. Moreover, the stations can be inserted and removed from the ring during normal ring operation.
As with any type of electromechanical device, the above communication systems are susceptible to fault or breakdown. The faults may occur in the communication link and/or at the nodes. Such faults often interrupt data transmission. The prior art describes several techniques for locating, reporting and correcting faults. For example, U.S. Pat. No. 3,564,145 describes a fault-locating device. However, before the fault-locating device can be utilized, the location and/or ordering of the stations on the loop must be known. The location and/or ordering information can be used to isolate error causing nodes and a break in the loop. Station location is of particular importance in a mobile and reconfigurable system where stations are added and removed, at will, from the communication link. In such systems once the fault is isolated the ring can be reconfigured and used for data transmission.